In the data sheet is written:
"Up to 400h standby, up to 20h while using it for talking"
This statement is true. "Up to" means it ranges somewhere between 0 - 400 standby and 0 - 20 for talking.
If you have a lame car with a maximum velocity of 80 km/h, you have to sell it with a maximum speed of "Up to 250 km/h" (If you drop it from an airplane).
Same for this mobile.

Useful might be a "test" under "realistic conditions". What I found out with the Roma I got was:
Standby time is ~ 3 days when new. 2 days after 1 year.
Active time (while using google maps and surfing) ~ 6h when new. 3h after 1h.

It brakes easly and there is no parts. My mom and neigbour have to buy NEW phones only because touch screen or display broke...It lags because 512mb or ram and battery dies after 15 hours of normal usage